1. Field of Invention
The present invention relates to certain color-formers, to their reactions with metal salts to form colored coordination compounds, and to imaging systems based thereon. The formation of colored coordination compounds can be employed to generate images and is important in the manufacture and use of pressure sensitive transfer papers for preparing carbonless copies.
The invention also concerns the admixture of such color-formers with N-(monosubstituted)dithiooxamides and/or N,N'-(disubstituted)dithiooxamides to form images of various colors, preferably black, upon application of appropriate pressure to pressure sensitive imaging constructions such as carbonless paper constructions.
2. Information Disclosure Statement
The use of coordination compounds to form imaging sheets has been important in the field of pressure sensitive transfer papers useful for preparing carbonless copies. The present invention provides color-forming compositions which, when complexed with transition metal ions, provide compositions which appear as intensely yellow colored complexes. This is accomplished in the present invention by the use of certain colorless aromatic substituted thiosemicarbazone compounds, aromatic substituted thiocarbohydrazone compounds, or of certain 2-alkylthio-N-[(2-hydroxyaryl)methylene]aniline imine derivatives, any of which provide an intense yellow color when individually complexed with cations of certain transition metals as, for example, nickel.sup.2+.
The aromatic substituted color-foraging compounds found to be useful in the present invention can be represented by the following formula: EQU Ar--CH.dbd.N--R I
wherein Ar is aryl and preferably is selected from the group consisting of phenyl, substituted phenyl, naphthyl, and substituted naphthyl bearing a hydroxyl group adjacent (i.e., ortho) or pseudo-adjacent (i.e., peri) to the site of attachment of the linking carbon atom attached to the nitrogen atom; and R is selected from the group consisting of: ##STR1## wherein R.sub.1 is a substituent comprising hydrogen, alkyl, cycloalkyl, and aryl (preferably phenyl, substituted phenyl, naphthyl, and substituted naphthyl); ##STR2## wherein R.sub.1 is as defined above; and ##STR3## wherein R.sub.2 is selected from the group of substituents comprising hydrogen, alkyl, cycloalkyl, branched alkyl, substituted alkyl, and heteroalkyl (e.g. alkyl ethers, alkylamides, alkylesters, and disulfides).
The class of compounds, represented by (i) are known as thiosemicarbazones, which can be prepared by the reaction of a thiosemicarbazide with an aldehyde or ketone. The thiosemicarbazides themselves are prepared by the reaction of hydrazine with an isothiocyanate. Thiosemicarbazones have been investigated as antituberculosis agents (see Wahab, A. Egypt J. Chem. 1978, 21, 403), as chemotherapeutic agents in the treatment of fungus infections (see Bhat, A. K., et al. Indian J. Chem. 1972, 10, 694; Bhat, A. K., et al. ibid. 1967, 5, 616), and as intermediates in the preparation of antihypertensive agents (see Tweit, R. C. U.S. Pat. No. 3,746,716 (1973). Some metal complexes of thiosemicarbazones have been investigated. These thiosemicarbazones had a free NH.sub.2 group at the 1-position (i.e., R.sub.1 .dbd.H). Coordination complexes with tellurium (see Singh, A. K.; Basumatary, J. K. J. Organomet. Chem. 1988, 346, 349) and mercury (see Chernova, T. N.; Lugovoi, S. V.; Chistota, V. D. Russ. J. Inorg. Chem. 1975. 20, 850) are known.
The class of compounds represented by (ii) are known as thiocarbohydrazones. The parent compound, thiocarbohydrazide, was first prepared by Stolle in 1908 (see Stolle, R.; Bowles, P. E. Chem. Ber. 1908, 41, 1099) by the reaction of hydrazine with thiophosgene or by the reaction of hydrazine with the hydrazine salt of dithiocarbazic acid. Hydrazine salts of dithiocarbazic acids were prepared earlier by Curtius (see Curtius, T.; Heidenreich, K. Chem. Ber. 1894, 27, 55). Stolle prepared thiocarbohydrazones by the reaction of thiocarbohydrazide with two molecules of an aldehyde. Kurzer and Wilkinson reviewed the chemistry and reactions of thiocarbohydrazide, carbohydrazide and related materials (see Kurzer, F.; Wilkinson, M. Chem. Rev. 1970, 70, 111). The reaction of thiocarbohydrazide with salicylaldehyde was reported by Gonzalez and coworkers who prepared 1,5-bis addition products (see Montana Gonzalez, M. T.; Gomez Ariza, J. L.; Garcia de Torres, A. An. Quim., Ser. B 1984, 80, 129). These workers were interested in the compounds as analytical reagents. They reported the Schiff bases exhibited several pK values, that they formed colors with transition metals and that the thiocarbohydrazide is very sensitive toward Fe(III) and In(III). Patil and coworkers studied the ML complexes (M=metal, L=ligand) of 1,5-bis(salicylaldehyde)-3-thiocarbohydrazones with various substituents in the 5-position on the aromatic ring. They prepared the 1,5-bis(salicylaldehyde), 1,5-bis(5-chlorosalicylaldehyde), 1,5-bis(5-methylsalicylaldehyde), 1,5-bis(5-methoxysalicylaldehyde)compounds and determined that the nickel(II) complex was tetradentate (see Patil, S. A.; Badiger, B. M.; Kulkarni, V. H. Acta Chim. Hung. 1983, 113, 129). In further work, Patil and coworkers also prepared cobalt(II), copper(II), as well as the mixed copper(II)/nickel(II) bimetallic complex. They reported such properties as elemental analysis, magnetic moment, and spectra of the compounds. They also gave the bactericidal activity of the ligand and the complexes (see Patil, S. A.; Kulkarni, V. H. Acta Chim. Hung, 1985, 118, 3). This group later investigated the Mn(II), Cr(III) and Fe(III) complexes with these ligands (see Shivaprasad, K. H.; Patil, S. A.; Patil, B. R.; Kulkarni, V. H. Acta Chim. Hung. 1986, 122, 169.
The class of compounds represented by (iii) are variously known as imines, anils, or Schiff's bases and include 2-alkylthio-N-[(2-hydroxyaryl)methylene]aniline compounds. They are prepared by the condensation of a 2-alkylthioaniline with a 2-hydroxy-aromatic aldehyde. 2-methylthio-N-(salicylidene)aniline has been prepared (see Goetz, F. J. J. Heterocyclic. Chem. 1968, 5, 509) and is known to form coordination complexes with metals such as Ni(II), Co(II), Pd(II), and Cu(II) (see Dunski, N.; Crawford, T. H. J. Inorg. Nucl. Chem. 1973, 35, 2707).
In none of the above cases have the color-formers or coordination compounds been used in any imaging process, nor has any reference to imaging been made, nor has any mention of their encapsulation been disclosed.
In order to be useful in an imaging construction, it is desirable that the color-former be capable of being encapsulated and of rapidly forming a stable colored image upon contact with the metal cation on the receptor sheet. That is, the transition metal complex should form nearly instantaneously, so that the image is rapidly formed as the stylus pressure is applied to the backside of the donor sheet. This will help ensure formation of an accurate, almost instantly readable, copy. The image should also be relatively stable so that it does not substantially fade with time.
The color-forming composition of the present invention can be readily microencapsulated by techniques known in the art, for example as described in G. W. Matson, U.S. Pat. No. 3,516,846. Pressure-sensitive record and/or transfer sheets can be provided as are known in the art.
When the yellow color-formers of the present invention are used in admixture with certain conventional dithiooxamide derivative transition metal complexing compounds, the light absorption properties of the individual complexes are additive. For example, when a yellow color-former of this invention is mixed with a magenta color-former such as an N,N'-(disubstituted)dithiooxamide a red color is obtained upon imaging. When a yellow color-former of this invention is mixed with a cyan color-former such as an N-(monosubstituted)dithiooxamide a green image is formed. When a yellow color-former of this invention is mixed with an effective amount of both an N-(monosubstituted)dithiooxamide and an N,N'-(disubstituted)dithiooxamide, or mixtures thereof, a black image is afforded. Thus, it is possible to absorb a sufficient portion of light in the visible spectrum so as to provide a neutral black color.
The chemistry and characteristics of metal complexes of dithiooxamide compounds have been used commercially and certain dithiooxamide compounds have been used in commercially available carbonless paper products. One successful type of carbonless imaging chemistry takes advantage of the fact that dithiooxamide compounds are encapsulable and react readily with many transition metal salts to form coordination complexes. Generally, these dithiooxamide compounds comprise symmetrically disubstituted dithiooxamide compounds and include N,N'-dibenzyl-dithiooxamide and N,N'-di(2-octanoyloxyethyl)dithiooxamide.
Generally, transition metal salts are used to form coordination complexes with dithiooxamides. Salts which have been employed in the preparation of carbonless image transfer products or constructions are those comprising cations having a +2 valence state. Compounds with nickel, zinc, palladium, platinum, copper and cobalt all form such complexes with dithiooxamides. Many of these coordination complexes are deeply colored.
Carbonless imaging constructions, or products employing this chemistry, generally involve placement of one reactant (i.e., one of the transition metal or color-former) on one substrate (for example, sheet of paper) and the other reactant (the one of transition metal or color-former not used on the first substrate) on a second mating substrate. The color-former and metal are maintained separated from contact and reaction with one another. This is typically accomplished by encapsulation of a solution of one of the reactants. Herein, the terms "encapsulation" and "encapsulated compounds" refer to microcapsules enclosing a liquid or a fill material therewithin.
Once rupturing pressure is applied to the construction, as from a stylus or business-machine key, the solution of encapsulated reactant is released, and a complex between the previously separated reactants is formed. In general, the resulting complex will, of course, form a colored image corresponding to the path traveled by the stylus, or the pattern of pressure provided by the key.
In one commercial product, the capsules on a first sheet (donor sheet) contain dithiooxamide (DTO) derivatives, and the mating sheet, sometimes referred to as the receptor sheet, contains a coating of selected salts of nickel. The encapsulated dithiooxamide ligands, in a suitable binder, are coated onto one face of the donor sheet; and, the metal salt, optionally in a suitable binder, is coated onto one face of the receptor sheet. Herein, the term "suitable binder" refers to a material, such as starch or latex, that allows for dispersion of the reactants in a coating on a substrate. In the case of a capsule containing sheet, a suitable binder will allow capsules to be readily ruptured under hand-held stylus pressure, or typical business machine key pressure. When the two coated faces are contacted such that the color-former and the metal salt can combine and react, a coordination complex forms and an image results. Typically, this occurs by transfer of the color-former to the site of the metal salt, i.e., transfer of the color-former from the donor sheet to the receptor sheet. The image, of course, forms on the receptor sheet.
In a preferred orientation, the encapsulated color-formers, in a suitable binder, are coated on the back of the donor sheet, sometimes referred to as a coated back (CB) sheet, and the metal salt, optionally in a suitable binder, is coated on the front of the receptor sheet, or coated front (CF) sheet. Again, in imaging, the two sheets are positioned such that the encapsulated color-formers on the donor (CB) sheet faces the metal salt coating on the receptor (CF) sheet. When pressure is applied to the uncoated surface of the donor sheet, i.e., the face not in contact with the receptor (CF) sheet, selected capsules rupture (i.e., those capsules corresponding to the pattern of applied pressure) with release of the color-former for transfer to the receptor sheet, forming a colored pattern due to complexation with the metal cation. In many applications the uncoated surface of the donor (CB) sheet comprises a form of some type. The stylus pressure is generated by means of a pen, pencil, or other writing instrument used in filling out the form. Thus, the image appearing on the receptor (CF) sheet is a copy of the image applied to the top sheet.
In another orientation, known as a self-contained carbonless paper, separate CB and CF sheets need not be used at all. In one type of self-contained carbonless sheet, both components may be incorporated within the paper during manufacture. One component, as for example the color-former, is encapsulated and the other component, as for example the developer, is within the paper but outside the capsules. Alternatively, one component (either the color-former or the developer) may be carried in the sheet and the other component (either the developer or the color-former) may be carried as a surface coating on the sheet. Other orientations are known.
In some applications, multiple form-sets have been used. These contain intermediate sheets having a metal salt coating on one side (i.e., the front side) and a coating with encapsulated color-former on the opposite side (i.e., the back side). Such sheets are generally referred to herein as "CFB" sheets (i.e., coated front and back sheets).
Due to the stoichiometry of the system (i.e., the metal salt is usually in excess since relatively little color-former is released and it is usually much less costly than the color-forming microcapsules), it is generally believed that the image formed on the receptor sheet, after stylus pressure is applied to break the capsules and release the color-former, results from the formation of a complex between one molecule of color-former and one atom of nickel having a +2 valence. The counterion of the positively charged transition metal is usually the conjugate base of a weak acid and may facilitate removal of the two protons from the color-former necessary for complexation with the M.sup.2+ cation. The loss of two protons from the color-former allows it to serve as a ligand with the metal (M).sup.2+ cation. The ligand.sup.-2- /metal.sup.2+ complex forms the colored image.
In commercial applications, generally, nickel salts have been preferred as the source for the transition metal cation. One reason for this is that nickel.sup.2+ salts form a deep color when complexed with the dithiooxamide ligands presently employed. The nickel salts are also substantially colorless, and thus do not alone impart color to the receptor (CF) sheet. A third reason is that nickel salts are relatively low in cost, by comparison to other transition metal salts that can be easily and safely handled and that form highly colored coordination complexes with dithiooxamides.
In some applications it is also desirable that the color of the complex be a deep, strong color that is not only pleasing to the eye, but that will exhibit good contrast with the paper for purposes of later reading and/or photocopying. Lack of these attributes has been one drawback with certain conventional carbonless paper arrangements, which use nickel salts complexed with disubstituted dithiooxamide ligands. The image imparted by the resulting coordination compound, under such circumstances, is generally magenta. The more "red" character the polymer complex exhibits, generally, the less contrasting and pleasing is the appearance. A dark, i.e., preferably black, blue, or blue-black, arrangement would be preferred, but previously such has not been satisfactorily obtainable. Recently, an attempt to achieve a blue or blue-black image by employing encapsulated N-(monosubstituted)dithiooxamides compatible with the transition metal chemistry described above was described in copending U.S. patent application Ser. No. 07/438,776, now U.S. Pat. No. 5,124,308. Preparation of these N-(mono-substituted)dithiooxamides is described in applicant's copending U.S. patent application Ser. No. 07/438,765 (now U.S. Pat. No. 5,041,654) incorporated herein by reference. Use of these N-(monosubstituted)dithiooxamides either alone or in admixture with N,N'-(disubstituted)dithiooxamides can result in a cyan, blue, or blue-black image. However, a neutral black image would be most preferred.
One attempt to prepare a neutral black image using transition metal coordination chemistry was disclosed in U.S. Pat. No. 4,334,015. It is disclosed therein that the combination of certain aromatic substituted hydrazones with dithiooxamides followed by encapsulation of the mixture provides a method of achieving a dark image. These hydrazones react with the metal on the receiving sheet to form intense yellow images. The yellow coordination compound thus formed, combined with the blue-purple image formed by the dithiooxamide (such as N,N'-(dibenzyl)dithiooxamide and/or N,N'-di(2-octanoyloxyethyl)dithiooxamide, results in an image that appears almost black to the observer.
Although this is a successful approach, the use of hydrazones disclosed in U.S. Pat. No. 4,334,015 still suffers from several drawbacks. The solubility of the hydrazones is not as great in the solvents generally used in the encapsulation process as are dithiooxamides. In addition, the initial image color of the coordination compounds formed of the mixture of these hydrazones with N,N'-(disubstituted)dithiooxamides is brown and only after some time does the red-black final image color form. Although somewhat more desired in some applications than the blue-purple coordination compound formed with the N,N'-(disubstituted)dithiooxamides alone, this mixture of yellow and blue-purple is a dark red-black rather than the preferred neutral black.
It was also noted in U.S. Pat. No. 4,334,015 that the color of capsules prepared from hydrazone compounds was pH dependent and their color may change from essentially colorless at low pH to yellow at pH greater than 9.5 to 10. It was further noted that this color change is rapid and reversible upon lowering of the pH. Papers can be divided into classes depending upon their methods of manufacture, treatment and sizing. Among these classifications are acidic and alkaline papers. More and more "alkaline paper" is being produced since it is considered to be long lasting and to have "archival" qualifies. Encapsulated hydrazones, when coated onto "alkaline paper" can form yellow colors, which on white paper and on some colored papers is undesirable.
The color-forming ligands generally useful in carbonless paper constructions should also be relatively nonvolatile, so that free color-former, which would result from any inadvertently ruptured capsule, does not readily transfer from the donor sheet to the receptor sheet and form undesired spots of imaged area. That is, so that without the specific assistance of stylus or key pressure, transfer is not readily obtained.
In conventional impact imaging constructions, the capsules can be inadvertently ruptured in steps such as processing, printing, cutting, packaging, handling, storing, and copying. In these situations inadvertent marking or discoloration (i.e., backgrounding) of the sheets results due to inadvertent capsule rupture and transfer of the encapsulated material to the mating sheet where color formation occurs. The amount of inadvertent backgrounding has been reduced in such products by the use of a color control coreactant distributed externally among the capsules. This coreactant is capable of reacting with the contents of the ruptured capsules before transfer of such contents to the receptor sheet and formation of an undesired image. See U.S. Pat. No. 3,481,759 which discloses that addition of a small amount of a metal salt such as a zinc salt to the dithiooxamide compound containing capsule coating prevents the formation of colored background. The zinc metal ion reacts with the dithiooxamide released adventitiously to form colorless coordination compounds.
The use of the invention disclosed in U.S. Pat. No. 4,334,015 in combination with that of U.S. Pat. No. 3,481,759 is not possible as zinc forms yellow coordination complexes with the hydrazones of the invention of U.S. Pat. No. 4,334,015. Thus, yellow color backgrounding still occurs on the backside of the sheet due to inadvertently ruptured capsules. It would be desirable to have a yellow color-former that could be successfully deactivated by the same method as that disclosed in U.S. Pat. No. 3,481,759. Then, the same method of deactivation of the yellow, magenta, and cyan color-formers released by inadvertent capsule rupture would be possible.
Another approach to formation of a black image employs an encapsulated mixture of an acid sensitive green-foraging leuco dye and a dithiooxamide color-former. The receptor sheet is formulated to contain phenolic type acids in addition to the transition metal salts. In this system, pressure imaging results in the release of both acid sensitive leuco dyes and dithiooxamide materials. The nickel salt in the receptor sheet reacts with the dithiooxamide to form a purple color and the phenolic acid in the receptor sheet reacts with the acid-sensitive leuco to form a green color. Together they generate a black image. This approach, while successful, has several disadvantages. Heavy coatings to the papers are required as two separate chemistries are involved. Another drawback of this approach is that the rates of reaction for the two chemistries are different and which results in images developing initially with a definite green or blue hue before turning black.
It is preferred that the color-former should be colorless, since the color-former is often encapsulated and coated onto the backside of a sheet, such as a form, which has printing on one or both sides thereof. This aspect is particularly important if the donor sheet comprises a top sheet for a stack of carbonless papers. Such sheets are often white, so that they can be readily identified as originals, can be readily photocopied, and can be easily read.
While the above-described preferred characteristics have long been desirable, they have not been wholly satisfactorily achieved with conventional reactants and conventional constructions. Suitable materials and arrangements for achieving the desired features described have been needed.